Extruder screw
The extruder screw design with recessed gears in the external gear reduces stress peaks and enhances torque transmission efficiency by offsetting meshing points from the hub edges, addressing plastic deformation issues in existing designs.
Patent Information
- Application Number
- JP2025123354
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-08-01
- Filing Date
- 2025-07-23
- Publication Date
- 2026-02-16
AI Technical Summary
Extruder screws experience undesirable plastic deformation due to high stress peaks at the hub gear connection, primarily caused by torsional stiffness differences between the screw shaft and elements, leading to inefficiencies in torque transmission.
The extruder screw design incorporates recesses in the external gear, offsetting the meshing points of the internal gear away from the hub edges, allowing for partial and controlled torque transfer, reducing stress peaks and enhancing torque transmission efficiency.
This design reduces stress peaks and plastic deformation, enabling improved torque transmission and increased durability of the gear connection by distributing load more evenly across the gear interface.
Smart Images
Figure 2026025955000001_ABST
Abstract
Description
[Technical Field]
[0001] The present invention relates to an extruder screw comprising a screw shaft and a plurality of screw elements removably mountable or mounted on the screw shaft, each screw element having a defined minimum axial length or a length corresponding to a multiple of the minimum axial length, the screw shaft having an external gear and the screw elements having an internal gear engaging therewith. [Background technology]
[0002] Such extruder screws, also known as plug-in screws, are known and allow for various extruder screw designs, allowing different screw elements, i.e., conveying, kneading, or mixing elements, to be arranged in different orders on the screw shaft, depending on the requirements. To facilitate installation, on the one hand, and to transmit the torque required for operation from the screw shaft, which is introduced via the extruder motor, to the screw elements, a shaft / hub gear is provided between the screw shaft and the screw elements. That is, the screw shaft is provided with an external gear, and the screw element, acting as a hub, is provided with an internal gear inside its bore, and the two gears mesh with each other. Extruder screws typically use a shaft / hub connection in the form of an involute gear according to the DIN 5480, DIN 5464, or ISO 4156 standards. This form-locking, symmetrical gearing allows for high torque transmission, while at the same time facilitating the assembly and disassembly of the screw elements.
[0003] Due to the high transmission torque, the gears are subjected to high loads, which can lead to undesirable plastic deformation of the gears, especially in the case of overload. The hub edge is particularly affected because high stress peaks occur due to stiffness variations within the connection. These stress peaks can occur on both flanks of the hub gear due to differences in torsional stiffness between the screw shaft and the individual screw elements. This stiffness difference causes the hub gear to contact the shaft gear not only on its front flank but also on its rear flank. In response to this, attempts have been made to provide steps on both axial sides of the hub internal gear flank, but this is very time-consuming because it must be performed as part of a separate post-processing step. Typically, the internal gear of a screw element is broached, but broaching only allows for flanks that extend linearly in the axial direction. Summary of the Invention [Problem to be solved by the invention]
[0004] SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved extruder screw.
[0005] To achieve this object, the present invention provides an extruder screw of the type described in the introduction, in which the external gear has a plurality of recesses extending circumferentially and offset over its axial length, forming individual gear rims extending along the circumference, the recesses being spaced apart by a minimum length, and the internal gear of each screw element protrudes into the region of the recesses at both axial ends, the ends of the internal gear not meshing with the external gear.
[0006] According to the present invention, the extruder screw is formed with a plurality of individual recesses extending along its circumference, so that a corresponding number of individual gear rims extending along its circumference are formed from the corresponding individual teeth or external gear segments. The recesses and gear rims are formed alternately with one another in the axial direction, and the axial spacing is determined by a predetermined grid. The recesses are separated from one another by a minimum length, i.e., the axes of all symmetrical recesses when viewed perpendicular to the longitudinal axis of the shaft are separated from one another by a minimum length. This minimum length corresponds to the minimum axial length of the screw elements slidable on the extruder shaft. As explained above, the screw elements may only have this minimum length in the axial direction, or may have a length that is a multiple of this minimum length, i.e., for example, two or three times the minimum length. The recesses, and thus the gear rims, are separated from one another in the axial direction along the screw shaft by this minimum length grid. For example, if the minimum length is 30 mm, the recesses are formed symmetrically along the screw shaft with respect to this 30 mm grid. However, this also means that the length of each gear rim, i.e. the axial length of the tooth or external gear section located inside the gear rim, or of its flanks, is shorter than the specified grid, i.e., a grid of 30 mm for example. As a result, each screw element sliding on the screw shaft has its internal gear meshing with the external gear of one gear rim (or several gear rims, in the case of screw shafts with a minimum length of twice or three times), but at its axial ends the internal gear does not mesh with the external gear, since these axial ends are located in the area of the recess. Gear meshing therefore does not take place over the entire length of the internal gear of the screw element, but only partially, i.e., in the part determined by the length of the external gear bearing the load in the respective gear rim.
[0007] This gear engagement, offset axially from the respective leading edges, has the advantage that the actual torque transfer from the screw shaft to the screw elements only occurs at a certain distance from the hub edge. Therefore, torque is transferred virtually only "inside the elements," which reduces potential stress peaks in the leading edge region, i.e., at the ends of the internal gear. The load on the gear ends is therefore reduced. This also allows for an overall increase in torque transmission, since the virtually "gentle" locally limited torque transfer does not require concern about excessive stress increases at the ends of the hub due to plastic deformation.
[0008] Each recess therefore excludes the respective ends of the internal gear of the screw element from gear meshing at the hub edge. Since the teeth of the internal gear have a corresponding radial length so that they extend deep into the groove between two teeth of the external gear, it is advantageous if the recesses extend all the way to the core of the screw shaft. This ensures that both ends of the internal gear do not mesh with the external gear, even in the event of possible twisting or other load-related shape changes.
[0009] There are various possibilities for the design of each recess. For example, the recess can have or be formed by a simple undercut, which divides the external gear into gear rims. However, this undercut can also form only a part of the recess, i.e. the recess has a recess area formed by the undercut, which is located, for example, in the center of the formed recess, as viewed in the axial direction. The undercut can have a length of, for example, 2 to 5 mm, and can extend, for example, to the core of the screw shaft.
[0010] It is particularly advantageous if the external gear comprises a central gear section with a maximum height between two recesses, which is flanked in both axial directions by lateral gear sections whose height is reduced by the formation of the recesses. The recesses are therefore not formed abruptly, for example by means of relief grooves with corresponding sharp gear rim edges, but by a gradual reduction in the tooth height of the external gear. The external gear has one central gear section, seen from the radial direction, where the gear meshing with the internal gear is at its maximum. The height of the external gear is then reduced on both axial sides of this central gear section, so that, on the one hand, the meshing height with the internal gear is reduced, and, on the other hand, the internal gear no longer meshes with the external gear at the ends where the height of the external gear is gradually reduced.
[0011] In this case, the height can decrease linearly, i.e., the external gear can decrease in height like a ramp. This height can decrease, for example, via an inclined ramp with a certain angle from the maximum gear height to the minimum gear height, for example, to the core of the shaft, or extend into the undercut groove. Instead of such a linear decrease in height, it is also conceivable, for example, for the tooth height to decrease convexly, i.e., to decrease with a slight outward curve, or to decrease wavy, i.e., to have a convex portion followed by a concave portion and then extend, for example, flatly into the undercut groove. Therefore, various tooth and therefore gear rim shapes are conceivable, which allow the height of the external gear to be reduced to form a recess.
[0012] As already mentioned, the high torque transmitted causes a constant elastic twist of the shaft over its length during operation, resulting in the leading and trailing flanks of each tooth of the internal gear coming into contact with the external gear. To ensure optimal contact between the internal gear and the external gear in the recessed region, where gear contact with the internal gear still exists over a certain length, as described above, an advantageous development of the present invention provides that the side gear sections have tooth flanks that are rounded or beveled on one or both sides. That is, the external gear sections in each gear rim are slightly rounded or beveled on one or both sides, as viewed circumferentially, so that despite the constant twist of the shaft, the internal gear makes very good contact without excessive stress peaks. This results in end relief for the teeth of the external gear. The fact that both the driving and trailing flanks have curved or beveled surfaces further enhances torque transmission.
[0013] It is particularly advantageous here if the curvature or bevel of the tooth flanks is designed to correspond to the expected helix angle of the screw shaft during operation, i.e., taking into account the expected helix angles that will occur during operation, which are only a few seconds to a few minutes locally over the length of such a gear rim, the curvature or bevel of the front and rear tooth flanks is not symmetrical but, so to speak, asymmetrical, so that the internal gear of the screw element can optimally contact the external gear, whose shape changes slightly due to torsion when a torsional load is applied.
[0014] As already explained, each recess can also have a recessed area in the form of an undercut groove, whereby the recessed area formed by the reduction in tooth height on both sides inside each gear rim extends into the undercut groove, i.e. merges into the undercut groove, so that the undercut groove reaches, for example, as mentioned above, all the way to the core of the screw shaft.
[0015] In one advantageous development of the invention, the height of the internal gear is reduced at both axial ends. Therefore, the internal gear also has a reduced height only in the area immediately adjacent to the two axial gear ends, i.e., at the transition to the hub edge. This advantageously prevents the internal gear from denting the screw shaft in the event of a possible tilt of the screw element (even if only slightly tilted), since the internal gear has no sharp gear edges at its axial ends. The height is preferably reduced via a curved surface, although a step is also conceivable. Here, both the curved surface and the step are provided solely to prevent denting, and therefore should be as short as possible in the axial direction.
[0016] According to the invention, the external and internal gears are preferably symmetrical, i.e., have the same flank angle or the same flank shape on both sides. The gears can be designed, for example, according to DIN 5480, which is a gear shape widely used in the field of extruder screws. However, for example, trochoidal gears are also conceivable, and basically, symmetrical gears can be used in each case.
[0017] Preferably, the gear rims and gears are produced without cutting. Therefore, the external gears and gear rims are not produced by cutting with broaching or milling tools and similar tools, but by rolling, for example. For this purpose, a corresponding profile rolling tool can be used, which has an image shape that is rolled onto the cold-formable screw shaft by rolling to form the gear rim and the adjacent recessed areas on both sides of it. Such a tool can, for example, form the gear rim and the corresponding recessed areas on both axial sides, and to produce a screw shaft with a corresponding number of gear rims, the screw shaft is successively offset axially by a corresponding number relative to a fixed tool, i.e., gear rims with associated recesses are rolled one by one. Of course, the tool can alternatively be moved axially.
[0018] In addition to the extruder screw itself, the present invention also relates to an extruder equipped with one or more extruder screws of the type described above.
[0019] If two or more extruder screws are used, these may preferably rotate in the same direction, although counter-rotation is also conceivable.
[0020] The present invention also relates to a method for manufacturing a screw shaft for an extruder screw of the above-mentioned type. This method is characterized by rolling an external gear onto the shaft body using a rolling tool without cutting, and the gear rim and the associated recessed areas are produced by intermittently offsetting the shaft body axially relative to the rolling tool. This method therefore provides for rolling the shaft body using a rolling tool without cutting. It is of course possible to use a profile rolling tool to form only one gear rim with recessed areas on both sides per axial position, but it is also of course conceivable to design the profile rolling tool to be correspondingly long so as to form two or more gear rims with corresponding recessed areas at one axial position of the workpiece machining, etc.
[0021] The shaft body itself is preferably made of a cold-formable material that is heat-treated for hardening after the gear rim is formed. This treatment is preferably carried out by aging, which achieves the significant increase in strength required for transmitting high torques. This material is therefore a corrosion-resistant, precipitation-hardening steel that is only annealed, so that it can be cold-formed on the one hand, and can be hardened by aging at a moderate aging temperature in the range of 400-600°C. Here, the hardness after aging should be, for example, 40-55 HRC.
[0022] Further advantages and details of the invention will become apparent from the following description of the embodiments and drawings. [Brief explanation of the drawings]
[0023] [Figure 1] 1 is a schematic diagram showing a screw shaft of an extruder screw according to the present invention. [Figure 2] 2 is a schematic diagram of an extruder screw according to the invention with a screw shaft according to FIG. 1. [Figure 3] FIG. 2 is an enlarged partial view of the screw shaft of FIG. 1 showing a gear rim with an associated recess. [Figure 4] FIG. 2 is an enlarged partial view of an extruder screw according to the present invention, showing an internal gear with reduced height. [Figure 5] FIG. 2 is a schematic diagram showing a second embodiment of an extruder screw according to the present invention. DETAILED DESCRIPTION OF THE INVENTION
[0024] FIG. 1 shows a screw shaft 1 equipped for an extruder screw 2 according to the invention, as shown in FIG. 2. The screw shaft 1 has, distributed over its length, a number of gear rims 3 spaced axially apart from one another at a defined spacing grid a, with recesses 4 extending into each gear rim 3 between them. Each gear rim 3 consists of a number of individual teeth 11 forming the gear rim in the circumferential direction. As already shown in FIG. 1, the height of each tooth 11 decreases towards its two axial ends, as will be explained in more detail below with reference to FIG. 3. In any case, this height decrease forms each recess 4 in association with a relief groove 5 located between the gear rims 3. According to a defined grid a, which corresponds exactly to the minimum axial length of the screw elements sliding on the screw shaft 1, a defined gear profile is provided on the external gear 6, which, viewed in the axial direction, is formed by a number of individual gear rims 3.
[0025] FIG. 2 shows a schematic diagram of an extruder screw 2 according to the invention, which consists of a screw shaft 1 and, in the example shown, a sliding screw element 18, although of course, a number of such screw elements 18 will be provided on the finished extruder screw.
[0026] The cross section shows an external gear 6 or gear rim 3, which can be seen to have a central gear section 7 with a constant tooth height. This central gear section is flanked on either side by two lateral gear sections 8, in which the tooth height decreases to form recesses 4, as shown in Figure 2. The gear sections 8, whose height decreases in an inclined, i.e. linear, manner in this example, extend into the relief grooves 5, as can be seen clearly in Figure 2.
[0027] The screw element 18 (which of course applies to all screw elements 18 slid on the screw shaft 1) comprises an internal gear 9 which meshes with the external gear 6. The internal gear 9 extends from one hub edge 10 to the other hub edge 10, i.e. over almost the entire axial length of the screw element 18. The screw element 18 shown here has a minimum length 1, i.e. the internal gear 9 corresponds to this minimum length 1 or, as will be explained below, is slightly reduced in height at the two hub edges 10.
[0028] In any case, FIG. 2 clearly shows that the internal gear 9 is fully engaged with the gear rim 3 or teeth 11 only in the region of the central gear section 7. The side gear sections 8 have a reduced height, resulting in a lower engagement height. As shown in FIG. 2, the gear sections of the internal gear 9 at the two axial ends of the internal gear 9, i.e., in the region of the hub edges 10, are no longer engaged with the external gear 6 or gear rim 3 but are unloaded and located in the recesses 4 and in the region of the respective relief grooves 5. Therefore, no torque transmission occurs at the hub edges 10, since there is no gear connection for transmitting torque. Rather, as the height of the external gear increases in the region of the side gear sections 8, the engagement continuously increases until, finally, maximum engagement is reached between the internal gear 9 and the external gear 6 in the region of the central gear section 7. This is where maximum torque transmission occurs. Because the hub edges 10 are excluded from torque transmission, no stress peaks occur at the hub edges 10. Stress peaks can cause plastic deformation of the internal gear in the region of the hub edge 10 in the case of high loads.
[0029] The gear rims 3 with the recesses 4 are, as mentioned above, arranged in a defined grid a and spaced apart from one another in the axial direction, which grid a corresponds exactly to the minimum length l of one screw element 18. This ensures that each sliding screw element 18, whether it has only the minimum length l or a multiple n of the minimum length l (total length = n x l), is always accommodated in the respective hub edge 10 in the region of the recesses 4 and therefore does not mesh with the external gear in the region of the hub edge 10.
[0030] FIG. 3 is a partial enlarged view of the gear rim 3. The gear rim 3 is composed of a plurality of individual teeth 11, each of which has a central gear section 7 in which the respective tooth 11 has a maximum gear height. On either side of the central gear section 7, two lateral gear sections 8 adjoin, where the gear height decreases down to the core 13 of the screw shaft 1, where the relief grooves 5 of the respective recesses 4 reach. The lateral gear sections 8 extend in an inclined manner, i.e., their height decreases linearly from their maximum height in the gear section 7 to their respective relief grooves 5. However, instead of an inclined shape with a linear decrease in height, a convex or wavy shape is also conceivable. As shown in FIG. 3, it is clear that the surface areas of the respective leading and trailing tooth flanks 14 and 15 also change, which necessarily results in a corresponding change in the contact area between the external gear 6 and the internal gear 9, which, locally, increases continuously up to the central gear section 7.
[0031] FIG. 3 shows an exemplary tooth 11a whose tooth flanks 14, 15 are flat up to the point where they drop into the relief groove 5. Furthermore, a tooth 11b is also shown, for illustrative purposes only, in which two tooth flanks 14, 15 have curved surfaces 16 at their ends, i.e., in the region of the side gear section 8, i.e., are not flat. This allows further optimization of the area where the contact surface between the internal gear 9 and each tooth 11b increases continuously, i.e., a gentle contact can be achieved during torque transmission. This is particularly true when the area with the curved surfaces 16 is designed taking into account the helix angle that occurs under load, i.e., the rotation about the longitudinal axis of the screw shaft 1. Therefore, the curved surfaces 16 are not symmetrical on the front flank 14 and the rear flank 15, but asymmetrical. This is because the individual teeth 11b extend at a minimum angle, i.e., with a minimum inclination relative to the longitudinal axis, depending on the helix angle. The configuration of the curved surface 16 (it is also possible to provide a flat bevel instead of the curved surface) can be adapted to this helix angle so that optimum contact is achieved under load between the internal gear 9 and the respective gear rim 3 or correspondingly designed teeth 11b. Naturally, it is also possible that only teeth 11a or only teeth 11b are provided to form the gear rim 3, but not a corresponding mixed form.
[0032] 4 is a partially enlarged view showing the region of the hub edge 10 of a screw element 18. The illustration shows an internal gear 9 with a clearly reduced height in the region of the hub edge 10, for which corresponding curved surfaces 17 (steps could also be provided instead of curved surfaces) are provided at the axial ends of the internal gear 9. These curved surfaces 17 are, of course, provided at both axial ends of the internal gear. As shown in FIG. 4, these axial ends or curved surfaces 17 are located in the region of the respective recesses 4 or undercuts 5. If the screw element 18 is slightly tilted at right angles to the longitudinal axis of the screw shaft 1 under load, these curved surfaces 17 prevent the hub edge from crashing into the screw shaft.
[0033] Finally, Figure 5 shows the configuration of an extruder screw 2 according to the invention, which comprises the screw shaft 1 according to the invention of Figure 1. The screw elements 18 shown here have a length, as shown, which corresponds, for example, to twice the minimum length l. It is clear that even in the case of this double-length screw element 18, the hub edge 10 and the axial end of the internal gear 9 located in this area are located in the area of the recess 4 or the undercut 5, so that even in the case of such a double-length screw element 18, the internal gear is excluded from torque transmission in the area of the hub edge 10. The same applies to even longer screw elements 18 which are multiples of the minimum length l.
[0034] To ensure that each screw element 18 is located in a defined axial position, the screw shaft 1 is naturally provided with a corresponding stop against which the first screw element abuts, which stops precisely position the first screw element relative to grid a, and thus each subsequent screw element, relative to grid a. This ensures that each hub edge 10, and with it each end of the internal gear 9, is located in the region of the recess 4 or undercut 5 and is therefore unloaded.
[0035] The profile of the gear rim of the screw shaft 1 is preferably formed by rolling using a profile rolling tool, by means of which the corresponding profile of the external gear 6 or the gear rim 3 together with the recess 4 is rolled into the metallic material of the shaft body before forming. A cold-formable steel is preferably used as material for the screw shaft 1, which can be correspondingly processed in the cold state using the profile rolling tool and can be appropriately hardened by a downstream heat treatment, in particular a simple ageing treatment, at least in the region of the external gear 6, so that the required hardness value is achieved in the region of the external gear 6.
Claims
1. An extruder screw comprising a screw shaft (1) and a plurality of screw elements (18) removably mountable or mounted on the screw shaft (1), each of the screw elements (18) having a predetermined minimum axial length (l) or a length corresponding to a multiple of the minimum axial length (l), the screw shaft (1) having an external gear (6), and the screw elements (18) having an internal gear (9) meshing with the external gear (6), 1. An extruder screw comprising: an external gear (6) having a plurality of offset recesses (4) extending circumferentially over its axial length, forming individual gear rims (3) extending along the circumference, the recesses (4) being spaced apart from one another by a minimum length (a); and an internal gear (9) of each screw element (18) protruding at both axial ends into the region of the recesses (4), the ends of the internal gear (9) not meshing with the external gear (6).
2. 2. Extruder screw according to claim 1, characterized in that the recess (4) extends to the core of the screw shaft (1).
3. 3. An extruder screw according to claim 1 or 2, characterized in that the recess (4) comprises a recessed area formed by a relief groove (5).
4. 10. An extruder screw according to claim 9, wherein the external gear (6) comprises, between two recesses (4), a central gear section (7) of maximum height, which is adjacent in both axial directions to lateral gear sections (8) of reduced height due to the formation of the recesses (4).
5. 5. An extruder screw according to claim 4, characterized in that the height decreases linearly, convexly or wavy.
6. 6. An extruder screw according to claim 4 or 5, characterized in that the lateral gear section (8) has rounded or beveled tooth flank surfaces (14, 15) on one or both sides.
7. 7. An extruder screw according to claim 6, characterized in that the curved or inclined surfaces (16) of the tooth flanks (14, 15) are designed according to the expected helix angle of the screw shaft (1) during operation.
8. 8. An extruder screw according to claim 3 or any one of claims 4 to 7, characterized in that each of the two lateral pawls (8) merges into one relief groove (5).
9. 10. An extruder screw according to any one of the preceding claims, characterized in that the height of the internal gear (9) is reduced at both axial ends.
10. 10. Extruder screw according to claim 9, characterized in that the height is reduced via a curved surface (17) or a step.
11. 10. An extruder screw according to any one of the preceding claims, characterized in that the external gear (6) and the internal gear (9) are symmetrical gears.
12. 10. An extruder screw according to any one of the preceding claims, characterized in that the external gear (6) and the recess (4) are manufactured without cutting.
13. 10. An extruder comprising one or more extruder screws according to any one of the preceding claims.
14. 14. Extruder according to claim 13, characterized in that when using two or more extruder screws (2), the extruder screws (2) rotate in the same or opposite directions.
15. A method for manufacturing a screw shaft for an extruder screw according to any one of claims 1 to 13, comprising:
1. A method characterized in that the external gear (6) is rolled onto a shaft body without cutting using a profile rolling tool, and the gear rim (3) and the associated recess are subsequently produced by axially offsetting the shaft body relative to the profile rolling tool.
16. 16. A method according to claim 15, characterized in that the shaft body is made of a cold-formable material which is subjected to a heat treatment for hardening after the formation of the gear rim (3).